1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotographic method, and to image forming apparatuses such as a copying machine, a printer and facsimile.
2. Related Background Art
A large number of image forming apparatuses using an electrophotographic method or an electrostatic recording method have heretofore been proposed. The schematic constitution and operation of the image forming apparatus will be described with reference to FIG. 13.
In the image forming apparatus shown in FIG. 13, when a copy start signal is inputted, a photosensitive drum 101 of a printer section is charged by a charger 103 to provide a predetermined potential. On the other hand, when an original placed on an original glass stand (not shown) of a reader section is irradiated by a unit integrally constituted of an original illuminating lamp, a short focus lens array and a CCD sensor and scanned, illuminating scan light reflected by an original surface is imaged by the short focus lens array and is incident upon the CCD sensor.
The CCD sensor is constituted of a light receiving section, a transmitting section and an output section, a light signal is converted to a charge signal in the CCD light receiving section, and successively transmitted to the output section in synchronism with a clock pulse in the transmitting section, the charge signal is converted to a voltage signal in the output section, and the voltage signal is amplified, lowered in impedance and outputted. An analog signal obtained in this manner is subjected to a known image processing, converted to a digital signal and sent to the printer section.
In the printer section, by scanning a laser beam L of a solid laser element turned on to emit light and off upon receiving the image signal by a polygon mirror rotating at a high speed, an electrostatic latent image corresponding to an original image is formed on the surface of the photosensitive drum 101. Subsequently, the electrostatic latent image on the photosensitive drum 101 is developed by a developing device 104 using a two-component developer in which toner and magnetic carrier are mixed, and the latent image is visualized as a toner image.
The toner image formed in this manner on the photosensitive drum 101 is electrostatically transferred to a transfer material P by a transfer charger 105. Thereafter, the transfer material P is electrostatically separated from the photosensitive drum 101 by a separation charger 106, and conveyed to a fixing device (not shown), and then the toner image is thermally fixed, and outputted as a printed image. For the photosensitive drum 101 from which the toner image is transferred, after adhering contaminants such as a transfer residual toner adhering to the surface are removed by a cleaner 107, the drum is repeatedly used for the next image formation.
In full-color image formation, from the viewpoint of high-speed printing, a so-called tandem type in which a plurality of sets of the photosensitive drum 101 and surrounding devices such as the charger 103 are arranged has started to be employed. As a transfer method, there has started to be used a direct transfer method which comprises: bearing the transfer material on a transfer belt formed of a resin belt or a rubber belt; conveying the transfer material to a plurality of photosensitive drums; and successively transferring toner images on the photosensitive drums in synchronism with the transfer material, or an intermediate transfer method which comprises: successively transferring the toner images on a plurality of photosensitive drums onto an intermediate transfer belt in synchronism with the intermediate transfer belt; and transferring the toner images to the transfer material from the belt.
Additionally, in recent years, as charging means for the photosensitive drum, because of advantages such as low ozone and low power, a contact charging device has been placed in practical use which brings a charging member with a voltage applied thereto in contact with the photosensitive drum 101 and charges the drum, and in the device, a magnetic brush is preferably used as the contact charging member in terms of the safe contact with the photosensitive drum.
In the magnetic brush type contact charging device, conductive magnetic particles are magnetically attracted and borne onto a magnet or a sleeve containing the magnet, the magnetic particles are stopped or rotated and brought in contact with the photosensitive drum, and the photosensitive drum is charged by applying a voltage to the contact charging member.
As the contact charging member, a conductive fur brush formed of conductive fibers in a brush shape, or a conductive rubber roller formed of a conductive rubber in a roller shape is also preferably used.
Particularly, when the contact charging member is used with a photosensitive drum including a surface layer in which conductive fine particles are dispersed on an ordinary organic photoconductor, or a photosensitive drum using an amorphous silicon photoconductor, the photosensitive drum surface is charged by an injection charging method, and it is possible to obtain a charging potential substantially equal to that of the direct-current component of a charging bias applied to the contact charging member on the photosensitive drum surface.
In the injection charging method, since an electric discharge phenomenon is not utilized as in a corona charging method, the complete ozone-less and lower power consumption type charging of the photosensitive drum is enabled, and the method has been noted.
In the image forming apparatus which uses the contact charging method, however, since the resistivity of the surface layer of the photosensitive drum lowers particularly under high humidity, the charge after the exposure is diffused, and a charge pattern is unsharpened. For example, when the image pattern requiring a high resolution is formed, the reproducibility of the pattern is deteriorated in some cases.
On the other hand, in the full-color copying machine, the prevention of forgery of images of paper money, and the like, and the tracking of output images needs to be realized. The tracking herein refers to the identifying of the copying machine type, machine type main body manufacture number, and the like from the output image. To realize this, as shown in FIG. 14, for example, a yellow toner is used to write a high resolution pattern for tracking into the image in an inconspicuous manner to such an extent that the essential original image is not influenced. Therefore, when the reproducibility of the high resolution pattern is deteriorated, the tracking of the output image to identify the apparatus having outputted the image cannot be performed.
As a factor of the above-described unsharpened charge pattern, when the photosensitive drum is contact-charged, a slight amount of discharge products (NOx) adhere to the photosensitive drum surface, the discharge products provide a low resistance (HNO3) under high humidity, the charge o the charge pattern formed on the photosensitive drum surface after exposure moves because of a low-resistance substance, and the charge pattern is disturbed in the phenomenon. When the fillers contained in the paper as the transfer material, such as talc, kaolin, and calcium carbonate adhere to the photosensitive drum surface, these deposits also provide the low resistance under the high humidity, so that the charge of the charge pattern moves and a smeared image is generated.
Moreover, in the injection charging method, the mode in which the latent image blur is generated differs, and the photosensitive drum as the charging object has a resistance layer on a surface layer. For example, when there is a charge transport layer right under the surface layer, the injected charge moves to the vicinity of a boundary surface between the surface layer and the charge transport layer, and the charge pattern of the latent image by the exposure is therefore formed in the vicinity of the boundary surface. Particularly, when the relative humidity is also high under high humidity, the volume resistivity of the surface layer is reduced, the charge is moved in the electric field caused by the charge pattern and the latent image blurs.
To avoid the problem by the resistivity drop of the surface layer under high humidity, when the resistivity of the surface layer is raised, the resistivity increases under low humidity, the injection charging becomes difficult, and the charging defect is disadvantageously caused.
To prevent the latent image blur, there is a method which comprises installing a heat source on the inner surface of the photosensitive drum and heating the photosensitive drum to lower the humidity of the photosensitive drum surface. When a plurality of photosensitive drums are arranged, however, a plurality of heat sources are also necessary, and the cost increase cannot be avoided.
An object of the present invention is to provide an image forming apparatus which can satisfactorily form a toner image for identifying the apparatus.
Other objects of the present invention will be apparent upon reading the following detailed description.